Particle image velocimetry (PIV) systems are well known for performing instantaneous, average velocity measurements of two-dimensional fluid flow fields. They provide quantitative information regarding the average flow velocity at multiple points in a plane at a given instant of time, which lends itself useful in the analysis of dynamic, transient flows.
PIV measurements, in the form of film or video images, are obtained by seeding the flow with small particles that are then repeatedly illuminated by a sheet of laser light at defined intervals in time, usually relative to some external trigger. Precise control of the time interval between laser sheet illuminations of the flow field is what produces the data for the system. There are several different methods for generating PIV images (e.g., double-exposed single-color laser illuminated, double-exposed dual-color laser illuminated). The method selected depends on the type of data reduction to be performed on the resulting image (e.g., autocorrelation, cross correlation). In the case of a double-exposed single-color image, the image is received and recorded via a recording medium, such as a CCD (charged coupled device) video camera, and analyzed by sub-regions using an autocorrelation based algorithm. The output of the autocorrelation algorithm is a group of peaks which represent possible average displacements of particle pairs in each sub-region of the image. These average displacements, when combined with the time interval (pulse spacing) between the laser pulses used in generating the image, provide the sub-region velocity information. The composite of all the sub-region velocities provide the flow field velocity.
One of the properties of autocorrelation based algorithms is that they map a physical displacement into a pair of symmetrical peaks in the result domain. These peaks are symmetrically located with respect to the center of the two-dimensional result domain. The magnitude of the displacement of each peak in a pair relative to the center of the result domain should be equal, but the direction each represents is 180 degrees apart. Without any a priori information relating to the general direction of a flow (i.e. for a given speckle pair, which of the two was illuminated first?) there is no information available to determine which peak in the pair is the proper directional result. This phenomena is referred to as velocity directional ambiguity.
Another problem related to double-exposed image processing results when the separation between particle pairs is small. With decreasing displacement of the image particles the associated peaks move closer to the center of the autocorrelation result domain. As the particle separation approaches zero, information is lost due to the presence of a singular central peak, which tends to be large enough in magnitude to swamp out the peak of interest.
One known method of compensating for these autocorrelation anomalies is to introduce a bias in the particle image data. This bias, in the form of a fixed spatial displacement, between data fields from successive flow field illuminations, produces a fixed velocity component which offsets the peaks in the autocorrelation domain and thereby facilitates peak selection and effectively increases the dynamic range of the instrument in the low velocity portion of its measurement range. These two items provide the means to remove the velocity directional ambiguity (including recirculating flows), as well as distinguishing low velocity flow components.
The prior art method of introducing this bias is disclosed in U.S. Pat. Nos. 4,866,639 and 4,988,191 to Adrian et al. In each patent the bias displacement is achieved by optically shifting, in real time, the beam trajectory of successive scattering site images to spatially offset the point of incidence of a succeeding return image on a recording surface (photographic film or video medium) from the incident point of a preceding return image.
In the '639 patent optical shifting occurs mechanically by reflection of the second (and succeeding) images from the surface of a rotatable mirror having selectably different angles of reflection for succeeding images. In U.S. Pat. No. 4,988,191 the return images pass through a birefringent crystal which, subject to an applied bistable electric field, produces a different angle of refraction for each image in a pair of images, thereby providing a comparable spatial offset between the point of incidence of each image at the recording surface.
In each method the real time trajectory of the image is altered to create the displacement at the incident surface. This displacement has a tolerance due to the tolerances associated with the optical and mechanical elements used to alter the beam trajectory. This tolerance translates to a velocity error which becomes increasingly significant as flow velocity decreases.